title: The muscle stretch reflex throughout the menstrual cycle.
abstract: BACKGROUND
  The significant sex disparity in sports-related knee injuries may be due to underlying differences in motor control. Although the development of sex-specific movement patterns is likely multifactorial, this study specifically focuses on the potential modulatory role of sex hormones.
  
  
  OBJECTIVE
  This study aimed to investigate the muscle stretch reflex (MSR) across the menstrual cycle. We hypothesized that the MSR would fluctuate throughout the menstrual cycle and that the lowest response would correspond with peak concentrations of estrogen.
  
  
  METHODS
  Nineteen healthy women age 18-35 yr participated in this study: 8 eumenorrheic women and 11 women taking oral contraceptives. Serum estradiol and progesterone concentrations, anterior knee laxity (AKL), and the MSR response of the quadriceps muscles were measured three times during the menstrual cycle.
  
  
  RESULTS
  The MSR response of the rectus femoris (RF) varied significantly across the menstrual cycle in both groups. Specifically, the RF MSR response was 2.4 times lower during the periovulatory phase when compared with the luteal phase (P = 0.007). The same trend was seen in the vastus medialis, but this did not reach statistical significance (P = 0.070). The MSR response of the vastus lateralis did not change significantly across the menstrual cycle (P = 0.494). A mixed model comparison did not show an association between endogenous concentrations of estradiol and progesterone, exposure to hormonal contraceptives or AKL, and the MSR response for any muscle.
  
  
  CONCLUSIONS
  Our results demonstrate that the RF MSR response varies throughout the menstrual cycle with the lowest response around the time of ovulation. Additional research is needed to clarify the exact relationship between sex hormones, AKL, and MSR response and to determine the specific origin of the change along the monosynaptic reflex arc.
authors.affiliation: 1Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Department of Physical Medicine and Rehabilitation, Northwestern University Feinberg School of Medicine, Chicago, IL; and 2Rehabilitation and Regenerative Medicine, New York Presbyterian Hospital, Columbia University Medical Center, Columbia University College of Physicians and Surgeons, New York, NY.
authors.forename: Ellen
authors.initials: E
authors.lastname: Casey
authors.forename: Farah
authors.initials: F
authors.lastname: Hameed
authors.forename: Yasin Y
authors.initials: YY
authors.lastname: Dhaher
mesh_terms: D000293:Adolescent; D000328:Adult; D001696:Biomechanical Phenomena; D004967:Estrogens / Q000097:blood; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology; D007719:Knee Joint / Q000502:physiology; D008597:Menstrual Cycle / Q000097:blood / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011374:Progesterone / Q000097:blood; D012018:Reflex / Q000502:physiology*; D055815:Young Adult
chemical_list: D004967:Estrogens; D011374:Progesterone